Apparatus for abrading articles

ABSTRACT

Apparatus for fine lapping of thin piezoelectric crystals to a desired resonant frequency characteristic includes a number of holders urging and lapping the crystals against a vibrating, relatively movable lapping plate bearing very fine abrasive particles. Probes are successively pivoted into engagement with selected holders, one at a time, during lapping to sample the instant resonant frequency of each of the vibrating crystals by coupling the crystals successively into a test circuit. While lapping of the other crystals continues, a crystal which has attained the desired frequency characteristic may be withdrawn with its holder and replaced by another holder and crystal.

United States Patent [72] Inventors Nicholas J. Mandonas 2,922,264 1/1960 Mushrush 51/58X Medford; 2,983,086 5/1961 LaChapelle 5 1/131X Paul A. Ryll, Haverhill, Mass. 2,984,951 5/1961 Coons et al. 51/131 [21] Appl. No. 766,735 3,027,689 4/1962 Merkl et a1. 51/165X [22] Filed Oct. 11,1968 3,063,206 11/1962 Meyerhoffet al. 51/165X [45] Patented May 25, 1971 3,088,247 5/1963 Mushrush 5 l/58X [73] Assignee Western Electric Company, Incorporated 3,097,458 7/1963 Richmond... 5 l/ 165X New York, NY. 3,350,817 11/1967 Kiso et al. 51/165 Primary ExaminerLester M. Swingle Attorneys-H. J. Winegar, R. P. Miller and S. Gundersen [54] APPARATUS FOR ABRADING ARTICLES 7 Chums 2 Drawing Flgs' ABSTRACT: Apparatus for fine lapping of thin piezoelectric U-S. crystals to a desired resonant frequency characteristic in- 51/58v 51/283 cludes a number of holders urging and lapping the crystals lllt. against a vibrating relatively movable plate bearing Fleld of Search very fine abrasive articles Probes are successively pivoted 1 129, 131, 283 into engagement with selected holders, one at a time, during lapping to sample the instant resonant frequency of each of [56] References cued the vibrating crystals by coupling the crystals successively into UNITED STATES PATENTS a test circuit. While lapping of the other crystals continues, 21 2,340,843 2/ 1944 Bailey... 5 1/283)( crystal which has attained the desired frequency characteristic 2,687,603 8/1954 White 51/283 may be withdrawn with its holder and replaced by another 2,884,746 5/1959 Rus et a1. 51/165X holder and crystal.

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TTuFg/vs 1 APPARATUS FOR ABRADING ARTICLES BACKGROUND OF THE INVENTION This invention relates to apparatus for abrading one or more articles and, more particularly, to apparatus for the fine lapping of thin piezoelectric crystals to adjust the crystals precisely to a predetermined resonant frequency characteristic.

In the production of piezoelectric crystals, for example, thin quartz crystals to be used in electronic systems for frequency control, the crystals are ground lapped or polished to impart a desired resonant frequency characteristic to each finished crystal. It is generally required that the frequency which is ob tained for each crystal so produced be within extremely close tolerances. Thus, a final, precision lapping operation on the crystals is most critical.

In order that an inordinate amount of time and effort not be expended in lapping piezoelectric crystals to value, it is desirable that a number of crystals be lapped simultaneously. However, in utilizing certain prior-art apparatus for lapping plural crystals simultaneously, it is necessary that lapping be periodically interrupted so that the individual crystals may each be tested for instant resonant frequency. Continued lapping cannot occur during the testing of the crystals. Thus, a series of intermittent lapping operations, followed by intermittent testing operations, is required. A crystal attaining the desired frequency value is removed from the apparatus after a testing operation and prior to a subsequent lapping operation. Those crystals which have not attained the desired frequency characteristic, on the other hand, must be lapped further. Such continued lapping cannot occur, however, during the intermittent testing operations or the crystal removal operations. It is clear from the foregoing that it would be desirable to provide methods and apparatus for fine lapping of piezoelectric crystals such that crystal lapping may be accomplished more efficiently, allowing lapping to occur during a greater percentage of the time during which the apparatus is utilized.

SUMMARY OF THE INVENTION An object of the invention resides in new and improved apparatus for abrading one or more articles, such as in the fine lapping of piezoelectric crystals to adjust the crystals precisely to a desired resonant frequency characteristic within close tolerances.

The invention contemplates the use of a number of electrically conductive crystal holders each urging one or more thin piezoelectric crystals against a lapping plate. A fixed housing retains the crystal holders in nest positions, from which each holder may be individually removed. The lapping plate is vibrated and is moved relatively to the crystal holders by a substantially noise-free, vibration-induced driving arrangement which will not interfere with crystal frequency measurement during lapping. Very fine abrasive particles on the lapping plate lap the crystals to alter their resonant frequency characteristics. The contemplated apparatus may be utilized to provide a final lapping step in a series of one or more abrading operations so as to impart a desired resonant frequency characteristic, within close tolerances, to the crystals.

A number of spring-loaded electrical probes are each individually engageable with a different crystal holder during the lapping operation. Engagement of a probe with a crystal holder couples the holder and its crystal or crystals into a test circuit which includes testing apparatus for measuring the instant resonant frequency of the held crystal or crystals. Such testing may occur with the lapping operation continuing, the engagement of a probe with a selected fixed crystal holder providing a precise indication of the instant resonant frequency of the held crystal or crystals being lapped between the selected crystal holder and the vibrating lapping plate. The vibration of the plate constitutes a complex vibration with components of multiple frequencies, including the resonant frequency of the crystal at any instant. Thus, the crystal is excited and voltage spike may be observed at the resonant frequency. This effect is utilized in measuring the resonant frequency characteristic at the particular instant.

Testing and lapping may occur simultaneously until a first crystal attains its desired frequency value. Sampling of the first crystal by the testing apparatus will indicate that the first crystal should now be withdrawn from lapping contact with the lapping plate. The apparatus is so constructed that the crystal holder which holds the first crystal may be removed manually from the fixed housing while movement of the lapping plate continues and while the other crystal holders urge other crystals against the lapping plate. As a result, the lapping of the other crystals is not interrupted by the removal of the first crystal and its holder. Likewise, the replacement of the holder with a new crystal to be lapped does not interfere with the continued lapping of the crystal held in other crystal holders. Thus, the structure of the apparatus permits its use in a most efficient manner to perform a new and improved method as follows: a plurality of crystals are lapped continuously and simultaneously until one crystal attains a desired frequency value, which value is indicated by testing during lapping. The lapping of only the crystal attaining the desired frequency value is terminated, so that such crystal may be replaced by a new crystal, while the lapping of other crystals which have not yet attained the desired value in continued. As a result, at any given instant during the operation of the apparatus, a number of crystals are simultaneously undergoing lapping.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a plan view of crystal-lapping apparatus constructed in accordance with the principles of the invention, showing a probe positioned to sample, with the aid of a test set, the instant resonant frequency of a selected crystal or group of crystals which are urged against a moving lapping plate by an associated one of several crystal holders each urging crystals against the lapping plate; and

FIG. 2 is an enlarged fragmentary side view, partly in section, illustrating the selected crystal holder urging the crystals against the moving lapping plate and showing a fixed plate mounting the crystal holder for removal therefrom and withdrawal of the crystals from the moving lapping plate when the test set indicates that a predetermined resonant frequency characteristic has been attained for the crystals.

DETAILED DESCRIPTION Referring to FIGS. 1 and 2 of the drawing, an apparatus for lapping articles such as thin quartz crystals 11 or other piezoelectric crystals includes an annular lapping plate 12 positioned between an annular upper plate or housing 13 and a base plate 14. The lapping plate 12 is positioned beneath the upper plate 13 with a pair of annular shoulders 16 and 17 of the lapping plate located below a pair of radially extending flange-sections l8 and 19, respectively, of the upper plate. A flat bottom surface of the lapping plate 12 rests on a fiat upper bearing surface of the base plate 14. The annular upper plate 13 is fixed against either horizontal or rotary movement by conventional mechanisms, e.g., keys, splines or studs, securing an outer edge of the upper plate to a fixed rim 21. The annular lapping plate 12 is not so constrained, but is free to move circumferentially beneath the fixed upper plate 13. The baseplate 14, in similar manner to the upper plate 13, is fixed against movement on a supporting frame 22.

A conventional vibration-inducing mechanism 23 is coupled to the supporting frame 22 for the baseplate 14. The vibrator 23 is effective to impart force components such that the freely movable lapping plate 12 will be caused to move circumferentially, at a relatively slow speed, in either a clockwise or counterclockwise direction between the fixed, annular upper plate 13 and the fixed baseplate 14 when the vibrator is energized. Alternatively, the lapping plate 12 may be oscillated about an intermediate position. An annular bottom ridge portion 24 below the annular shoulders 18 and 19 of the upper plate 13 extends downwardly between the shoulders 16 and 17 of the lapping plate 12 so as to allow only rotary circumferential movement of the lapping plate upon energization of the vibrator 23. The upper plate 13 is constructed of relatively lightweight dielectric material in order that 1) no substantial frictional drag force will interfere with rotary movement of the lapping plate 12 and (2) electrical isolation will be provided by the lapping plate 12.

A groove 26 extends into a top surface of the fixed upper plate 13 and runs circumferentially thereabout. A number of spaced cylindrical apertures or bores 27 are arranged along the groove 26 and extend vertically through the upper plate 13 between the top and bottom surface thereof. Each of the apertures 27 in the fixed upper plate 13 constitutes a nest for housing a cylindrically shaped crystal holder 28 therein. Each crystal holder 28 is constructed of electrically conductive material and may individually be manually placed into or removed from its respective aperture 27. The operator need only grasp that portion of the outer cylindrical surface of the crystal holder 28 which normally extends vertically within the groove 26 to insert or remove the holder.

Each crystal holder 28 includes one or more appropriately shaped, shallow recesses 29 (three shown in FIG. 1) for each holding a crystal 11 therein. The crystals may be held in place in the recesses 29 by an adhesive or by other suitable means, e.g., mechanical fastening mechanisms or vacuum. Each crystal extends slightly outwardly from a recess 29. Thus, an exposed surface of the crystal will contact a raised annular lapping surface 31 of the lapping plate 12 when the associated crystal holder 28 is nested in its aperture 27 with the crystal facing downwardly, as shown in FIG. 1. Very fine abrasive particles 32, e.g., silicon carbide or aluminum oxide, may be placed onto the lapping surface 31 of the lapping plate through one or more of the apertures 27 when the associated crystal holder or holders 28 are removed. As a result, upon energization of the vibrator 23 to impart circumferential, rotary or oscillatory movement to the lapping plate 12, the surfaces of the crystals 11 which are in contact with the raised lapping surface 31 will be finely lapped by the abrasive particles 32. The abrasive particles may be suspended in any suitable carrier fluid, e.g., oil. The crystals will be continuously urged against the abrasive particles 32 on the lapping surface 31, as lapping continues, by the weight of the removable crystal holders 28.

As stated previously, the fixed upper plate 13 is composed of a dielectric material, e.g., a lightweight plastic. Thus, the crystal holders 28, which are made of an electrically conductive material, will be electrically isolated from each other when nested in the apertures 27. The lapping plate 12, the baseplate 14, and the abrasive particles 32 are all formed of electrically conductive materials. A number of electrically conductive probes 33 are pivotally mounted on a support member 34, which is composed of a dielectric material. The support member 34 may, alternatively, be integral with the upper plate 13. Each probe 33 is attached to a lever 35 and is associated with a different aperture 27 in the upper plate 13. When a lever 35 is pivoted, the attached probe is moved into engagement with a crystal holder 28 positioned in the associated aperture. The probe and lever are preferably lightly spring loaded in order to maintain such engagement once the probe is pivoted. The spring loading obviates any necessity for frequent position adjustments for the probe as the lapping of a held crystal or crystals 11 results in downward movement of the crystal holder 28. The engagement of a pivoted probe with a crystal holder will complete an electrical circuit from a test set 40 through a wire or other conductor 36, the pivoted probe 33, the engaged crystal holder 38, one or more crystals residing in one or more recesses 29 in the engaged crystal holder, the lapping plate 12 and the abrasive particles 32 thereon, the baseplate 14 and a wire or other conductor 37. The conductors 36 and 37 may be separately arrayed wires or may be conductive elements within a single coaxial cable.

The test set 40 includes a precision frequency selector or tuner 41 and a highly sensitive voltmeter 42. When one of the probes 33 is engaged with a crystal holder 28, the test set 40 will sample the instant resonant frequency of one or more crystals 11 being lapped by the lapping plate 12 beneath the holder. if more than one crystal is held by the holder 28, each crystal will provide an individual frequency indication through use of the test set 40. The test set will indicate that a sampled crystal 11 is close to or has attained the desired resonant frequency and, in the latter case, that lapping of the sample crystal should be ceased.

It should be noted that the vibrator 23 performs simultaneously several functions. Thus, the vibrator will induce a slow rotary or oscillatory movement of the lapping plate 12 to lap the crystals 11. At the same time, the use of the vibrator 23, rather than an induction motor, to drive the lapping plate 12 will eliminate a potential source of stray frequency noise. Such noise would interfere with the precision frequency measurement required in ascertaining the instant resonant frequency characteristic in a selected crystal 11 undergoing lapping. Additionally, the vibrator will excite the selected crystal 11 at its instant resonant frequency, the vibrator causing the lapping plate 12 to vibrate with components of vibration at multiple frequencies including a component corresponding to the resonant frequency of the crystal at any given instant during lapping. Thus, the vibrated crystal 11 will generate a voltage signal at its instant resonant frequency. The frequency of this signal will change continuously, but slowly, as the selected crystal is lapped to decrease its thickness. Manipulation of the frequency selector or tuner 41 until a relatively large voltage spike is observed on the meter 42 will indicate that the instant resonant frequency of the crystal 11 being tested has been located by the tuner 41. The instant resonant frequency value may then be read directly from the tuner.

In the operation of the apparatus of FIGS. 1 and 2, it is assumed that all of the crystal holders 28 are initially withdrawn from the apertures 27 in the fixed, annual upper plate 13. Very fine abrasive particles 32 may, thus, be introduced onto the raised lapping surface 31 of the lapping plate 12 through one or more of the open apertures 27.

Crystals 11 are now inserted into the recesses 29 in the crystal holders 28, to be retained therein during a fine lapping operation, with an exposed surface of each crystal extending outwardly from its recess 29. Next, the crystal holders 28 are inverted and are inserted manually one into each aperture 27 in the fixed upper plate 13. The inversion of the crystal holders results in the exposed surfaces of the held crystals 11 being placed into contact with the abrasive particles 32 .on the raised lapping surface 31.

The vibrator 23 is now energized by the operator closing a switch (not shown) in a line to a power source. Vibration is imparted to the supporting frame 22, the baseplate 14, the lapping plate 12 and the crystals 11 in the holders 28. The vibration induces a slow rotary or oscillatory movement of the lapping plate 12 circumferentially of the annual upper plate 13. The vibrating crystals 11 are, thus, lapped on the surfaces thereof which extend outwardly from the holders 28 and contact the abrasive particles 32 on the lapping surface 31. The weight of the crystal holders 28 urges the crystals into continuous contact with the abrasive particles during lapping. The crystal holders 28, while constrained by the walls of the aperture 27 against movement circumferentially of the fixed plate 13, are free to rotate about their own axes in the apertures 27 so that the crystals 11 held by each holder may be more evenly lapped. A convex or domed tip or nose 43 of each probe 33 is positioned to extend when pivoted, into a similarly shaped cavity 44 in each associated crystal holder 28. The domed tips 43 act as pintles to permit continued crystal holder rotation during testing engagement between a probe and a crystal holder.

As lapping continues, the instant resonant frequencies of the various vibrating crystals 11 being lapped are sampled successively. Thus, a first probe 33 is pivoted into engagement with its associated crystal holder 28 (FIG. 2) and the instant resonant frequencies of the held crystal or crystals are measured. Such measurement occurs as the frequency selector or tuner 41 is adjusted until a surge of high voltage is observed on the meter 42. The surge of voltage indicates that the frequency to which the selector 41 has been adjusted corresponds to the instant resonant frequency of one of the crystals 11 being lapped beneath the erl'gaged crystal holder 28. The frequencies of any other crystals 11 held by the engaged crystal holder may be'similarly identified. A precise, sequential monitoring of the individual crystal frequencies is possible due both to l a relative absence of stray frequency noise through the use of the vibrator 23 rather than an induction motor in driving the lapping plate 12 and (2) the relatively slow rate of change of the resonant frequency characteristic in the crystals undergoing precision lapping at slow speed using very fine abrasive particles.

The first probe 33 is next pivoted out of engagement with its crystal holder and a second probe 33 is pivoted to sample the instant resonant frequency of the crystal or crystals held by a second holder 28. All ,of thec'rystals are tested successively in similar manner an their instant resonant frequencies are noted. T

The crystal ll havirig a tested resonant frequency characteristic closest to a desired final value is now sampled once again. Other crystals having approximately equal frequencies may also be retested. The crystal ll closest in frequency to the desired value is monitored continually until the desired value is attained. The crystal holder 28 retaining this crystal is now grasped by the operator and is lifted from its aperture 27 in the fixed upper plate 13 to remove the crystal from contact with the lapping plate 12..A spare crystal holder (not shown) with one or more crystals retained therein may be inserted immediately into the empty aperture 27 to replace the removed crystal holder 28. If more than one crystal 11 is retained in the removed crystal holder sequential testing of the individual crystals in the removed holder will indicate which of the crystals has attained the desired resonant frequency value. Movement of the lapping plate 12, meanwhile, continues. Thus, lapping of other crystals is not interrupted.

A second crystal 11, now closest in frequency to the desired value, is identified and sampled. The procedure outlined above in connection with the first crystal is repeated for the second crystal. Meanwhile, the first crystal may be removed from its recess 29 in the crystal holder 28 and a new crystal inserted into the recess 29. The first crystal holder 28 may be set aside as a spare, while monitoring of the second crystal continues.

Similar operations may follow, in order to lap any number of crystals 11 to the desired resonant frequency characteristic. Any number or all of the crystal holders 28 may be nested in the apertures 27 at any given time. Movement of the lapping plate 12 may be continued 50 long as further lapping of any crystal is required. The number of apertures 27 is preferably not overly large and the speed of movement of the lapping plate 12, which carries the very fine abrasive particles 32, is relatively slow. Thus, neither the number of crystals to be monitored nor the rate of change of crystal frequencies makes individual sampling and removal of crystals during the lapping operation too difficult for the operator to accomplish.

Briefly, reviewing the method of the invention, articles, such as piezoelectric crystals, are abraded by urging the articles against a lapping plate or other abrasive surface while moving the abrading surface with respect to the articles. for example by vibrating the abrading surface. A characteristic of a selected article, which characteristic, such as resonant frequency, is altered by abrasion, is measured simultaneously with the abrading steps. When the characteristic value attains a predetermined levehthe article is removed from contact with the abrading surface while abrasion of at least one other article continues. This method of abrading may be performed using the exemplary apparatus of FIGS. 1 and 2, or with any other apparatus for abrading articles, suitably modified to whatever extent may be necessary to allow individual testing of articles undergoing abrasion.

It is to be understood that the above-described method and apparatus are simply illustrative of one embodiment of the invention. Another embodiment of the apparatus might utilize only a single probe, the single probe being mounted to be moved into engagement successively with each of the crystal holders 28 in the apertures 27, e. g., by movement of the probe along a trackway on the rim 21. Certain embodiments might use other known frequency measuring circuits in place of the exemplary test set 40. Likewise, plural test sets may be used to measure the instant resonant frequencies of plural crystals simultaneously. Additional embodiments might be provided for lapping, polishing, grinding or otherwise abrading other types of articles to any given thickness tolerance or smoothness, for example, in order to alter any measurable characteristic of the article or articles. Many other modifications may be made without departing from the invention.

We claim:

1. In apparatus for abrading an article having a vibration response characteristic which is determined by the thickness of the article to alter the vibration response characteristic to a predetermined value:

an abrading member mounted for movement upon vibration thereof;

article holder means for urging the article against said abrading member so that the thickness of the article will be decreased and the vibration response characteristic of the article will be altered upon movement of the abrading member;

means for vibrating said abrading member to abrade the article and vary the vibration response characteristic; and means responsive to vibration during the abrading for indicating the attainment by the article of the predetermined value of the vibration response characteristic.

2. ln apparatus for lapping a piezoelectric crystal:

a fixed holder for the crystal;

a relatively movable lapping member positioned to contact the crystal on said holder;

means for moving said lapping member with respect to the holder during said contact to reduce the thickness of the held crystal, said means comprising means for vibrating the lapping member to excite the crystal; and

circuit means, completable through the held crystal and rendered effective when completed upon excitation of the crystal with movement of said lapping member, for measuring the instant resonant frequency of the excited crystal.

3. ln apparatus for lapping piezoelectric crystals:

at first fixed holder for a first crystal;

a relatively movable lapping member positioned to contact the first crystal on said holder;

means for moving said lapping member with respect to the holder during said contact to reduce the thickness of the first crystal;

at least one additional relatively fixed crystal holder located adjacent said movable lapping member in position to lap another crystal between the additional crystal holder and the relatively movable lapping member; means for removably mounting each of said holders in the fixed position thereof such that at least one of the crystal holders may be removed during movement of said lapping member; and

circuit means completable through each held crystal and rendered effective during movement of said lapping member for measuring the instant resonant frequency of each crystal through which the circuit means is completed.

4. ln apparatus for lapping crystals, as set forth in claim 3:

said crystal holders each being composed of an electrically conductive material;

said holder-mounting means being composed of a dielectric material so as to electrically isolate the crystal holders from each other; and

said circuit means including an electrical conductive probe mounted for engaging one of said crystal holders to measure the frequency of a crystal held thereby during lapping.

5. ln apparatus for lapping piezoelectric crystals to vary the resonant frequency of the crystals:

a plurality of holders for the crystals;

a lapping plate mounted for movement;

means removably mounting each of said holders fixed with respect to said lapping plate and positioned to urge the crystals held by the holders against the lapping plate for lapping the crystals upon movement of the lapping plate;

means for vibrating and moving said lapping plate to excite and lap the crystals; and

means operated during the vibration and movement of said lapping plate for measuring the instant resonant frequency of a crystal being lapped.

6. In apparatus for lapping piezoelectric crystals to adjust the crystals to a predetermined resonant frequency value:

a fixed plate having a pair of opposite faces and having a plurality of apertures extending through the fixed plate between said faces;

a lapping plate having a lapping surface and mounted for movement with the lapping surface maintained adjacent one of said faces of the fixed plate;

individual holder means removably positionable in said apertures of the fixed plate for releasably holding a crystal and urging the crystal against said lapping surface of the lapping plate to be lapped upon movement of the lapping plate;

means for moving the lapping plate to lap the held crystals;

and

means selectively monitoring the instant resonant frequency of the held crystals during continued lapping for indicating that a held crystal has attained the predetermined value.

7. ln apparatus for lapping piezoelectric crystals, as set forth in claim 6, said lapping plate moving means comprising:

means for vibrating the lapping plate during movement to excite the held crystals. 

2. In apparatus for lapping a piezoelectric crystal: a fixed holder for the crystal; a relatively movable lapping member positioned to contact the crystal on said holder; means for moving said lapping member with respect to the holder during said contact to reduce the thickness of the held crystal, said means comprising means for vibrating the lapping member to excite the crystal; and circuit means, completable through the held crystal and rendered effective when completed upon excitation of the crystal with movement of said lapping member, for measuring the instant resonant frequency of the excited crystal.
 3. In apparatus for lapping piezoelectric crystals: a first fixed holder for a first crystal; a relatively movable lapping member positioned to contact the first crystal on said holder; means for moving said lapping member with respect to the holder during said contact to reduce the thickness of the first crystal; at least one additional relatively fixed crystal holder located adjacent said movable lapping member in position to lap another crystal between the additional crystal holder and the relatively movable lapping member; means for removably mounting each of said holders in the fixed position thereof such that at least one of the crystal holders may be removed during movement of said lapping member; and circuit means completable through each held crystal and rendered effective during movement of said lapping member for measuring the instant resonant frequency of each crystal through which the circuit means is completed.
 4. In apparatus for lapping crystals, as set forth in claim 3: said crystal holders each being composed of an electrically conductive material; said holder-mounting means being composed of a dielectric material so as to electrically isolate the crystal holders from each other; and said circuit means including an electrical conductive probe mounted for engaging one of said crystal holders to measure the frequency of a crystal held thereby during lapping.
 5. In apparatus for lapping piezoelectric crystals to vary the resonant frequency of the crystals: a plurality of holders for the crystals; a lapping plate mounted for movement; means removably mounting each of said holders fixed with respect to said lapping plate and positioned to urge the crystals held by the holders against the lapping plate for lapping the crystals upon movement of the lapping plate; means for vibrating and moving said lapping plate to excite and lap the crystals; and means operated during the vibration and movement of said lapping plate for measuring the instant resonant frequency of a crystal being lapped.
 6. In apparatus for lapping piezoelectric crystals to adjust the crystals to a predetermined resonant frequency value: a fixed plate having a pair of opposite faces and having a plurality of apertures extending through the fixed plate between said faces; a lapping plate having a lapping surface and mounted for movement with the lapping surface maintained adjacent one of said faces of the fixed plate; individual holder means removably positionable in said apertures of the fixed plate for releasably holding a crystal and urging the crystal against said lapping surface of the lapping plate to be lapped upon Movement of the lapping plate; means for moving the lapping plate to lap the held crystals; and means selectively monitoring the instant resonant frequency of the held crystals during continued lapping for indicating that a held crystal has attained the predetermined value.
 7. In apparatus for lapping piezoelectric crystals, as set forth in claim 6, said lapping plate moving means comprising: means for vibrating the lapping plate during movement to excite the held crystals. 